Fossil fuel is currently still the major source of functionalized carbon skeleton for the chemical and energy industry. However, as prices of oil increase and fossil fuel reservoirs deplete, new demands for molecules from renewable resources will be created, and biomass conversions using bio-refineries are expected to play a more significant role in the future.1 One important advantage of using biomass is that the carbon source is renewable as it ultimately comes from atmospheric carbon dioxide, harvested photosynthetically to form simple sugars and their polymers such as cellulose. Furthermore, since cellulose is the most abundant organic molecule on earth, it has recently become the principle focus of emerging renewable carbon-fuel technology.2 The earliest form of biomass conversion was from lignocellulose, an abundant material comprising of cellulose, hemicellulose and lignin, and its earliest use was the commercial production of ethanol through a fermentation process in the early 20th century3. Cellulose and hemicellulose were regarded as of particularly high importance because their glucose monomers were the source of carbon in the production of ethanol, although other types of monosaccharide are also present in hemicellulose. However, more complex functionalized carbon skeletons are needed if biomass was to replace fossil fuel as the raw material for precursors in synthetic polymer and pharmaceutical production.4 
One of the most promising chemical building blocks, among those previously considered, is 5-(hydroxymethyl)furfural (HMF), first discovered by Dull, G. in 1895.5 HMF holds the potential to securing future energy and chemical needs as it is obtainable from renewable sources such as non-food crops, avoiding competition with food. HMF is synthesized mainly by the dehydration of monosaccharides, although dissacharides or polysaccharides such as sucrose, cellobiose, inulin and cellulose can be used as starting materials with a necessary initial hydrolysis step for depolymerisation.6 
One example of the use of HMF is its conversion to 2,5-dimethylfuran (DMF) whose energy content of 31.5 MJ L−1 is comparable to that of gasoline (35 MJ L−1) and 40% greater than that of ethanol (23 MJ L−1).7 Other uses include its conversion to other important molecules in the chemical industry such as levulinic acid (LA)—a precursor to plastics, 2,5-diformylfuran (DFF)—an intermediate to pharmaceuticals, 2,5-furandicarboxylic acid (FDA)—a precursor to polyester, and many more8.
Fortunately, the most important source of HMF remains to be that from cellulose—the major component of non-food crops (Scheme 1).

Research in this area is relatively new and there are still many challenges to overcome. The first challenge to overcome in using cellulose-based feedstock is its dissolution. Cellulose has a highly crystalline structure with extensive network of intra- and inter-molecular hydrogen-bonds within and between parallel chains, respectively, rendering it insoluble in most solvents.9 Many attempts have been made to solve this problem and the most successful methods are those using (1) NaOH and urea at low temperatures, and (2) ionic liquid (IL).10 However, since monosaccharide dehydration is catalysed by acids, IL is the better solvent for a one-pot HMF production from cellulose after hydrolysis to glucose. Furthermore, ILs have many advantages over other solvents.11 
The second challenge to HMF production from cellulose is that, while the dehydration of fructose to HMF is known to occur readily12, the dehydration of its glucose monomer after hydrolysis proceeds slowly due to the slow first-step isomerisation to fructose (Scheme 1).
One explanation for this is the fact that there is a much lower abundance of acyclic isomers for glucose compared to fructose.13 Glucose can form stable ring structures, slowing down its isomerization to fructose and thus conversion to HMF. Many attempts have been made to address this bottle-neck, using either Brönsted acid or Lewis acid catalysts.14 However, Brönsted acids catalyse more unwanted side-reactions due to its strong aqueous acidity.13b 
In recent years, catalysts for carbohydrate dehydration have undergone a remarkable process of evolution, and several Lewis acid catalysts have been reported6. However, the yields remain lower, and unwanted side-reactions remain higher, than are practically desirable for many of these Lewis acid catalysts reported—with the exception of transition metal chlorides such as GeCl4, SnCl4, CrCl3 and CrCl2.12,15 Anhydrous CrCl3 is currently the best known catalyst for glucose dehydration to HMF. Discovered by Zhao et al., the group reported HMF yields of 68-70% for the dehydration of glucose in 1-ethyl-3-methylimidazolium chloride ([EMIm]CI) IL solvent at a temperature of 100° C. for 3 hours, at 6 mol % catalyst loading with respect to glucose (Scheme 2).12 This result was also supported by work done by others.16 However, chromium chloride salts are known to be toxic and environmentally hazardous, limiting its practical scale-up in industrial processes17.

On the other hand, boron-based catalysts such as boronic acids may be considered to be non-metals and are known to possess low toxicity, as evidenced by their applications in medicine. Furthermore, from an environmental perspective, boronic acids will degrade to the relatively benign boric acid in air and aqueous media, although the fate of the rest of the molecule depends on the nature of its substituent. Despite these advantages, however, no study has been done on the use of boronic acids as catalysts for HMF production, even though it is known to be a Lewis acid by virtue of its vacant p-orbital. Khokhlova, E. A. et al. reported on the mechanistic study of B(OH)3, B2O3 and PhB(OH)3 in carbohydrate conversion to HMF using NMR studies, although no measurement of HMF yields were made.18 As early as 1974, Scott, R. W. et al. used boric acids in the dehydration of mannose, while in 2010, Stahlberg, T. et al. discovered B(OH)3's role in the dehydration of glucose to HMF with a reported yield of up to 41.5% in [EMIm]Cl at a temperature of 120° C. for 3 hours, and at 100 mol % catalyst amount with respect to glucose19,20.
Boronic acids' entry into biological and medicinal applications only started in the early 1990s. Of its biological applications, the most relevant for discussion are its role in glucose sensors and transmembrane transporters, both of which require the selective interaction of boronic acid with 1.2 and 1,3 diols on the glucose molecule to form boronate ester complexes with 5- and 6-membered rings, respectively (Scheme 3b). It was observed that the formation constant for diol boronate anion complex (Ktet) was much higher than the formation constant for diol boronic acid complex (Ktrig), highlighting its preference for the anionic form (Scheme 3a). One reason given was that the neutral diol boronic acid complex deviated from its ideal trigonal planar bond angle of 120° due to an O—B—O angle compression to 113°, while the anionic complex provides a closer match to the ideal tetrahedral geometry bond angle.

However, there is still a need to provide further and improved catalysts to be used in the conversion of saccharides such as glucose or cellulose in HMF and thereby provide a more efficient and economic process for HMF production.